The present invention relates to an apparatus for measuring the azimuth and elevation of an object.
The invention also concerns a method for measuring the azimuth and elevation of an object.
The invention is applied to high-resolution measurement of bearing angles (azimuth and elevation) over a large solid angle (typically over a hemisphere) corresponding to an object emitting, reflecting or relaying a marker signal. The method makes it possible to perform continuous tracking of the object.
Direction finders, that is, devices indicating an exact bearing of a selected object have been developed. Bearing angles indicated by a two-coordinate direction finder are generally designated by the azimuth (bearing angle across the horizon) and the elevation (bearing angle above or below the horizon). The location of the "horizon" of the two-coordinate direction finder is a matter of convention, permitting it to be different from the horizontal plane. In contrast to directionally selective receivers which receive a minimally corrupted signal, the characteristic feature of the direction finder is its capability to indicate an exact bearing of the real object. A directionally selective receiver can utilize signals reflected from the environment, too, while these signals represent false signals to the directional finder.
Direction finders are generally designed for the identification of either electromagnetic or mechanical vibrations. Of these two applications, those associated with electromagnetic emissions are more conventional.
An implementation frequently used in radio engineering is an antenna with high directional gain, capable of being rotated in two dimensions and used for the location of an object. Directional gain is typically achieved by means of a parabolic reflector.
For the purpose of measuring the bearing of the object and tracking of a mobile object, the surroundings of the object are scanned both in the azimuth and elevation planes. If the incoming signal is found to be stronger in any direction, the sensor is pointed to that direction. The scanning is performed by deflecting the main beam either electrically or mechanically. Tracking methods are described in several textbooks, for instance, in Antenna Engineering Handbook by Richard C. Johnson and Henry Jasik, McGraw-Hill Co., New York (2nd Edition 1984), Chapter 40-4.
A rotatable scanning sensor necessitates precision mechanical engineering. Because the entire elevation control machinery is moved along with the change of azimuth, the entire mechanical construction becomes heavy, thus necessitating the use of high-power motors and sturdy support constructions.
The pointing accuracy of the tracking sensor is linearly proportional to the aperture of the sensor and inversely proportional to the wavelength of the signal used. Therefore, the implementation of a high-precision sensor results in a high wind load and consequentially heavier mechanical constructions. The pointing accuracy is so decisive for the implementation of the rest of the system that this characteristic must be determined first during the design stage of the system.
Both azimuth and elevation tracking is implemented by means of an identical oscillating-scan method of the main beam, which in practice invariably results in interference between the signals of the different bearings. Interference by itself results from, e.g., timing errors, movements of the object during measurement or, alternatively (particularly in radio engineering), changes in the polarization plane of the transmitter resulting in a misinterpretation of the associated voltage differentials by the tracking algorithm.
The nature of the tracking algorithm dictates that the object is never coincident with the maximum gain axis of the scanning beam. The oscillating scan of the beam causes an amplitude modulation of the received signal at the oscillating-scan frequency.
A sensor with a high gain is incapable of tracking sudden movements of the object, since the pencil beam of the antenna is narrow and the mechanical system steering it is heavy. Due to the nonideal behavior of the tracking algorithm, a large steering movement in one dimension can result in a unintended change in the other tracking angle, thereby causing the loss of the target from the beam.
Loss of target becomes a major problem if the sensor is mounted on a mobile platform. The apparent movements of the object are actually the inherent movements of the sensor not yet compensated by the steering mechanism of the sensor.
The direction finder can be implemented with no moving parts as an interferometer, that is, using a sensor array from which the signal of each sensor element is processed separately. The bearing to the object is determined by comparing the transit time differentials of the signals received by the sensors against the physical spacings of the sensors. Interferometers are treated in several comprehensive textbooks, e.g., S. Haykin (Ed.): Array Signal Processing, Prentice-Hall Inc., Englewood Cliffs, N.J. (1985).
Because the signals from each of the sensors are processed individually, the sensors must have a sufficiently wide aperture to cover the entire solid angle of interest. A typical sensor in radio engineering is a ground-plane antenna, which covers almost a hemisphere.
Due to the small directional gain of the sensors, the interferometer has a short range detection capability. This is because directional sensors are unfit if a single set of sensors is used for tracking over a wide solid angle. Neither can the gain be improved by increasing the number of the sensors because time differentials cannot be measured from sensors having excessive noise which make it impossible to utilize coherent summing of the sensor signals.
If directional sensors are used for increasing the tracking range, they must be pointed to different parts of the solid angle of interest in order to cover the entire solid angle. The width of the sensor beam is inversely proportional to its gain, thus necessitating the addition of the sensors in linear proportion to the desired gain.
Generally, the beam patterns of the sensor elements are elevation-dependent. A ground-plane antenna provides no gain at low elevation angles and at the zenith.
A sensor array has always a direction in which its projected area is small. In order to avoid mutual shadowing, the sensors are typically arranged in a single plane which is parallel with the platform. If the target approaches in the same plane, the sensor array looses its direction-finding capability, because the projected area (effective aperture) in that direction is virtually zero.
Due to the nondirectional character of sensors in an interferometer, they provide no discrimination against surrounding interference even if the interference originates from a direction totally different from that of the target.
Aircraft and low-frequency ground station technology also utilizes a method, which is based on a plurality of sensors with different beam patterns. The mutual magnitude ratios of the received signals can be used for solving the bearing (Johnson and Jasik, Chapter 40-3).
Amplitude comparison is used principally because the sensor fields in this method can be constructed extremely compact. The sensor apertures are narrow and the directional accuracy of the sensors is low.
A successful amplitude comparison requires a high signal-to-noise ratio. The beam patterns of the sensors must be selected according to the direction finding method, instead of directional gain, thus making it impossible to attain sensor gain or directional discrimination of interference in this method.